This invention relates generally to expanded and compressed scale measuring devices, and more particularly to instrumentation which includes electronic circuits for supplying output voltages having the necessary expanded/contracted incremental characteristics for directly driving an inexpensive conventional instrument movement such as a milliammeter or galvanometer.
In the past, a number of techniques have been employed for obtaining expanded scale readings over predetermined subranges within a larger measurement range, in order to achieve increased accuracy and resolution. Prior devices have involved meter movements including tailored pole pieces, tapered cores, or special magnets whereby magnetic field gradients of unique character were produced within the air gap in which the field coil was movable. In such devices, equal increments of current would produce different deflections depending upon the rotary position of the coil within the air gap.
While such movements operated in an acceptable fashion, there were a number of distinct drawbacks which became apparent. The manufacture of tapered pole pieces is both costly and difficult to control. As a result, substantial variation was encountered between different units of the same type. In addition, it was found that such instruments were somewhat unstable in that the exact points of expansion and/or contraction were extremely difficult to control. In manufacturing runs of 50 or 100 instruments, considerable variation was found to occur from unit to unit. Attempts to calibrate each completed instrument separately helped to overcome the above disadvantages, but resulted in increased assembly time and higher overall cost. In addition, such movements employed no negative feedback, and were thus susceptible to somewhat greater inaccuracy than other devices employing feedback.
Still other systems involved servo feedback loops employing servo motors driving non-linear potentiometers through suitable gear trains and the like. Typically, the indicator was connected to be driven through the same gear train as that which drove the potentiometer. Negative feedback was provided from the potentiometer to the input of the amplifier which powered the servo. Depending upon the nature of the non-linear potentiometer, equal voltage increments over the range being measured or monitored resulted in unequal increments of feedback voltage derived from the potentiometer, thus resulting in non-linear scale readings. Problems were encountered with such systems, in that there was a tendency for the devices to "hunt" about a quiescent point, this resulting in objectionable noise and minute oscillating movements of the servo motor and gears. In addition, the gear trains were generally complex, noisy, and expensive to manufacture and maintain. Some variation was usually encountered between the non-linear potentiometers from unit to unit, this resulting in slight variations in calibration from one device to the next.
Still other systems involved non-linear amplifiers which employed a multiplicity of switching diodes in a feedback loop around the amplifier. The arrangement was such that as the output of the amplifier attained different values, successive diodes were rendered conductive, thus altering the net closed loop gain of the amplifier and resulting in expanded or contracted readings within part of the range of signals being monitored. One device of this type is known as a logarithmic amplifier, which provides an output voltage proportional to the log of the input voltage. Such amplifiers have wide application in the field of spectrum analysis, where logarithmic scales are employed for the vertical axis on a cathode ray tube. These amplifiers generally operated in a satisfactory manner, but employed a number of variable resistors in series with the switching diodes in order to enable variation in the cut-in point of each diode to be achieved. In addition, problems were frequently encountered with temperature drift of the diodes, wherein the switch-on voltage typically varied by one or two millivolts per degree centigrade of temperature change. Moreover, it has been found to be difficult to effectively compensate for this drift in such amplifiers.